Jo May 6, 2024
To estimate the lifetime of welding joints of steam pipes working at high temperatures and high pressures is of great importance for the stability and maximum economic efficiency of steam power plants. Long-term exposure of heat-resistant steels at high temperature leads to deterioration in the mechanical properties because of the changes in the microstructure. Alloying elements and microstructures significantly affect the heat resistance in the welds of low alloy heat resistant steel at high temperature.
In order to illustrate the changing nature of heat resistance, Ri Won Jun, a researcher at the Faculty of Materials Science and Technology, investigated the mechanical properties (ultimate strength, yield strength, elongation, hardness, etc.) of pipe materials and their welding joints with respect to the content of elements in carbides, their morphology and change in the metallic structures. Also, he conducted an experiment by using zirconium instead of vanadium for electrodes to evaluate the microstructure of deposited metal, mechanical properties at room temperature, metallic structures analysis, carbide analysis, scale resistance, short-term creep strength and long-term creep strength.
As a result, he drew the following conclusions.
First, the metallographic study showed that Zr series has less migrant tendency of the grain boundary than V series at high temperatures for a long time. And the comparison of mechanical properties with hybrid carbides showed that Zr series is superior to V series in all indices.
Second, the experiment for oxidation inclination at high temperature showed that the value of scale oxidation of Zr series is 1.4 times higher than that of V series, indicating that Zr series is more stable at high temperature.
Third, the metal deposited by the stick electrode coated with Zr has no ferrite segregation and the primary structure is finer than that with V.
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Jo Apr 29, 2024
Investment casting is widely used to make complex castings with high dimensional accuracy at low cost. The impellers of centrifugal pumps have many thin-walled regions and structures of uneven thickness in parts, so shrinkage defects can occur in these areas.
Also, their complex internal cavities cause excessive variations in the velocities of molten metal flowing into the shell mold. This results in gas and non-metal inclusion defects in castings. Therefore, it is important to avoid these casting defects occurring in investment casting of impellers.
Kim Yu Chol, a section head at the Faculty of Materials Science and Technology, has succeeded in making centrifugal pump impellers with no defects by optimal design of runner/riser system and mold tilt angle with ProCast software, thus enhancing the quality of castings and reducing production hours. The sprue was used as a kind of riser to feed shrinkage of casting and enhance casting yield. The shell mold was tilted at various tilt angles to eliminate gas and inclusions during pouring.
You can find the details in his paper “Improvement of quality and yield for investment casting of centrifugal pump impeller by tilting mold and optimizing runner/riser system” in “The International Journal of Advanced Manufacturing Technology” (SCI).
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Jo Apr 26, 2024
Preparation of copper nanoparticles have depended on liquid-phase reduction, where copper sulfate as raw material is dissolved in deionized water and a certain amount of dispersant is added before they were heated under stirring to reach reduction reaction temperature and reducing agent is dropped and mixed to produce copper nanoparticles.
Then, mixing reducing agents takes long while the reduction rate of copper ions is very fast (reduction reaction is already initiated before required amount of reducing agent is mixed). This results in non-uniform nucleation-crystalline growth, thus copper nanoparticles unhomogeneous in size being obtained.
After the reduction reaction, the copper nanoparticles dispersed in liquid phase are separated from the liquid phase using a high-speed centrifuge, which are diluted again in deionized water, and centrifugation washing process is repeated several times, so oxidation by atmospheric oxygen is easily achieved.
Kim Song Chol, a researcher at the Faculty of Materials Science and Technology, prepared relatively uniform-size copper nanoparticles by thoroughly mixing the reactants (copper salt solution + reducing agent solution) at room temperature in the liquid-phase reduction process (based on the fact that reduction reaction never occurs even when copper sulfate aqueous solution and reducing agent solution are mixed below 40ºC) and heating to the reaction temperature under stirring, allowing simultaneous and homogeneous nucleation of crystalline nuclei to be formed in the reaction system.
In addition, he realized separation and wash of produced copper nanoparticles from aqueous solution without a centrifuge, and instead introduced volatile organic solvent into reaction system to encapsulate the produced copper nanoparticles. By doing so, he ensured their surfaces are protected as soon as the copper nanoparticles are produced, and also simplified the separation and washing process.
The proposed technique can reduce the production cost of copper nanoparticles and increase the rate of recovery, particle size homogeneity and oxidation stability. In addition, it is a great potential for practical applications as producing copper nanoparticles as there is no need for expensive high-speed centrifuges. It is also useful for mass production of copper nanoparticles as lubricating oil additives.
You can find more information in his paper “A new method to improve homogeneity and oxidation stability of Cu nanoparticles for lubricant additive in liquid phase reduction process” in “ Materials Research Express ” (SCI).
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Jo Apr 24, 2024
Generally, less active metals (Cu, Ni, Zn…) are electrolyzed in aqueous liquid, but highly active metals such as rare earth elements and light metals are not. This is because hydrogen is gassed on electrode before deposition of metal in aqueous solution. Electrodeposition of rare earth elements is only possible by fused salt electrolysis at high temperature. Therefore, researchers have studied electrodeposition of one or two rare earth elements in molten salts and special organic solvents.
One of the major advantages of ionic liquids in electrodeposition is that electrochemical processes are carried out at low temperatures close to room temperature. This can reduce the energy loss of electrochemical processes that must be carried out at high temperatures. Therefore, some researchers have used various ionic liquids for electrodeposition of various active metallic elements at low temperatures. However, the liquids are highly hygroscopic and thus, an inert gas-filled space must be provided to prevent moisture for the elctrodeposition, which makes them difficult to be applied to industrial applications.
Use of 1-ethyl-3-methyl-imidazolium fluoroborate (EMIMBF4) ionic liquids has the potential for electrolysis of rare earth elements with no special conditions.
An Hyo Song, a researcher at the General Assay Office, has proposed a new method for the co-electrodeposition of dysprosium and terbium using EMIMBF4 ionic liquid.
The experiments demonstrated that the electrodeposition of dysprosium and terbium ions in EMIMBF4 ionic liquids is an irreversible and simultaneous process. He used the measured data to ensure that the electrolyte composition and diffusivity are reasonable, and newly determined the kinetic diffusion coefficients of these chemical reactions from cyclic voltammetric and chronoamperometric analyses.
For more information, please refer to his paper “ Simultaneous electrodeposition behavior of dysprosium and terbium in 1-ethyl-3-methyl-imidazolium tetrafluoroborate ionic liquid” in “ Journal of the Indian Chemistry Society” (SCI).
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Jo Apr 23, 2024
300M steel is a typical ultra-high strength steel with very high strength, excellent fracture toughness, good fatigue strength and stress corrosion resistance, and thus it has found expansive applications in the manufacture of key components of military and civil aircrafts.
300M steel can be formed into complex-shaped parts such as landing gears which are one of the four core components of aircraft, as well as into sheet or rod. However, deformation of 300M steel should be performed at elevated temperatures because of its high strength and limited ductility.
In recent years, some scholars have established constitutive models to predict the hot deformation behavior in ultra-high strength steels. Unfortunately, few studies on phenomenological models for 300M steel have been conducted or reported, except for the AT model.
Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has developed and improved sc-AT, m-JC and KHL models to describe the hot deformation behavior of 300M ultra-high strength steel.
First, he conducted isothermal uniaxial compression tests under various thermo-mechanical processing conditions.
Second, by using the experimental data, he developed the AT, m-JC and KHL models for 300M steel, and evaluated the predictability of the models.
Based on the analysis of the cause of large deviation, he further improved the m-JC and KHL models, and verified the prediction accuracy of the five constitutive models by using standard statistical parameters.
He drew the following conclusions.
The m-JC and KHL models show relatively good predictability at the reference conditions. The sc-AT model exhibits the highest R2 value of 0.9971 and the lowest AARE value of 3.57 %. In view of the prediction accuracy and computation complexity, the improved versions of m-JC and KHL models are preferred models among the phenomenological constitutive models he studied.
The details are found in his paper “Development and Improvement of Several Phenomenological Constitutive Models for Thermo-mechanical Processing of 300M Ultra-high Strength Steel” in “Journal of Materials Engineering and Performance” (SCI).
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Jo Apr 22, 2024
Solid polymer electrolyte water electrolysis (SPEWE) is considered to be a promising way of producing “green” hydrogen at lower temperatures without use of fossil fuel or emission of greenhouse gas. In addition, electrolyser with proton exchange membrane (PEM) is expected to be used extensively due to its higher efficiency and current density, and use of distilled water as reactive agent.
Currently, the most widely used proton exchange membrane material is Nafion due to its moderate proton conductivity and chemical stability. But owing to the high cost of Nafion membrane, researches for alternative membranes based on hydrocarbon polymer and composite membrane have been prompted. Sulfonated polyether ether ketone (sPEEK) membranes are potential candidates to take the place of Nafion membranes as they are cheap while exhibiting good properties. Their conductivity and water uptake increase with increasing sulphonation degree are superior to Nafion. In addition, the dependence of conductivity with water content is stronger than in the case of Nafion. Especially, the conductivity of sPEEK at high levels of hydration is far superior to that of Nafion.
Jang Tae Il, a researcher at the Nano Physics Engineering Institute, has prepared ZrO2-sPEEK composite membrane with proper solvency by dimethyl formamide (DMF) and mechanical strength, and studied the factors for preparation of MEA. He chose ZrO2 based on the following motivations; firstly, chemical stability of ZrO2-sPEEK composite membrane can be enhanced by acid resistance of ZrO2 and secondly, good hydrophilicity of ZrO2 can improve or at least not worsen the proton conductivity of composite membrane.
He concluded from his study that the reasonable content of ZrO2 in the proposed composite membrane is 12wt%.
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